A granulator is used to reduce the size of plastic or other materials to particles small enough to be used in reprocessing, disposal, etc. This size reduction is accomplished through the use of knives to cut the material into smaller pieces. A granulator has one or more stationary (bed) knives and two or more rotating (rotor) knives. When a rotor knife passes a stationary knife, a cutting action is produced, assuming that a piece of material is located between the knives. A half cylinder containing a large number of perforations (screen) is located directly below the knives to control the size of granulated particles leaving the cutting area. Particles are forced to remain in the cutting area until they are small enough to fall through the screen.
Granulators of this basic type have been manufactured for over 30 years. The traditional approach to design has been to make the knife large enough to accommodate several round holes so that bolts can be inserted through these holes for the purpose of clamping the knife to its mating structure. Knives are currently manufactured from a solid piece of tool steel, which is usually "hardened" using a thermal heat treatment process. Once knives become dull, they are typically removed from the machine, resharpened, and installed back into the granulator. This requires frequent readjustment of the clearances between the rotor and bed knives each time knives are resharpened. The cost of resharpening, along with the removal and installation labor, can be substantial. A typical knife can be resharpened about five times before it becomes too small to be effective and must be disposed of. When a knife is disposed of, it still contains approximately ninety percent of its original material.
Another problem with conventional knives is that as they become dull the quality of the granulated material suffers. Dulled knives tend to "beat" or "hammer" the material rather than cut it. As a result, the amount of dust or "fines" in the granulate increases. Users wish to reduce the amount of fines to a minimum, because the dust is objectionable from an environmental standpoint, it causes problems in material transfer systems, and usually results in waste.
If knives could remain sharp for a longer period of time, the costs associated with resharpening and changing the knives would be reduced. The quality of the granulated material would also be improved.
Extending the life of a cutting edge requires a more durable material; i.e., a material that wears better and is not brittle. Such materials do exist. The real problem arises when one attempts to use these exotic materials in a conventional knife configuration. The cost per pound of these materials is significantly higher than conventional tool steels, and this approach is just not economically attractive.
Over the years there have been many attempts to improve the life of granulator knife cutting edges. The development of better tool steels has caused some increases in knife life, compared to common chrome-vanadium-steels (CVS), but at a higher cost. Better life can be obtained from a knife made from hardened D-2 tool steel, but the cost is about 40 percent higher. Still greater life can be achieved by treating a knife surface with a flame sprayed tungsten or titanium carbide coating. Knife life can triple, compared to that of a chrome-vanadium-steel knife, but not without a substantial increase in cost.
Other coatings on conventional knives have been tried with the goal of improving edge life. Titanium nitride coatings, which have dramatically improved the life of such things as drill bits, have been tried. Although some improvement was achieved, the economics are not attractive.
During the 1970's, most granulator manufacturers offered a knife consisting of strips of tungsten carbide attached to mild steel knife bodies. The tungsten carbide is very hard and would enhance the wear characteristics, while the majority of the knife is made from low cost mild steel. Although this approach met with some success, the difficulties in joining the two metals made this a very expensive process, and therefore not a good solution to the knife wear problem. In a similar fashion, D-2 tool steel strips were attached to mild steel bodies. This approach also did not prove to be reliable or economical.
Metallurgically bonding a small amount of titanium carbide particles suspended in a tool steel matrix to a conventional tool steel body in an attempt to achieve an economical solution to knife wear has been attempted. The product is machinable before heat treatment, after which it requires grinding. While this approach sounds promising to some, the initial cost of a set of knives is very high.
Another approach tried was to apply a cladding to the knife tip area on a mild steel knife base. After the very hard stellite material was applied, the final knife cutting edge had to be machined through an electrical discharge machining (EDM) process. The economics of this approach did not prove to be acceptable.
Several granulator manufacturers experimented with knives and rotors having intermeshing teeth or grooves, in an attempt to reduce the amount of time required for knife adjustment. By moving each knife "forward" by one tooth when it is reinstalled, by theory further adjustment becomes unnecessary. This approach required that knives be resharpened to exacting dimensions so that the cutting edges are at predicted locations after being re-installed in the granulator. The additional cost of building the teeth in the rotors and knives, plus the added care required during sharpening has also caused this approach to be unsuccessful.
A small, reversible, and expendable knife was developed, see U.S. Pat. No. 3,981,337. Its complex shape is produced by extrusion from a high durability proprietary steel, and the final critical features are produced by grinding. This knife is positioned and retained to its mounting surface by a clamp. While the economics of the knife itself appear to be good, there are two problems. First, the life is not significantly better than what is achievable using a conventional knife made of D-2 tool steel. Secondly, the knife requires complex and precise features on both its mounting surface and its clamp. The costs associated with dedicated fixuring and tooling required to manufacture these components make this knife a less attractive solution.
The present invention overcomes these prior art problems. A knife design is provided which uses considerably less exotic material when compared to prior art products and is easier to manufacture.
The cutting edges of the knife being in cross-section, the acute angles of a trapezoid and thus two cutting edges are located opposite each other. This knife is characterized by at least two holes, one near each end of the knife. These holes have precise diameters and are very accurately located equidistant from the two cutting edges and from each other. These holes are used as locating features in the knives. The knife is placed on a mounting surface (either the rotor seat or the bed knife clamping bar), which contains protruding locating pins. These pins are precise in both diameter and location and have the function of accurately positioning the knife edges in the granulator. This pin arrangement ensures the accuracy of each cutting edge location.
Once positioned, each knife is rigidly held in place by a clamping bar. Bolts are inserted through holes in the clamping bars, and once the bolts are tightened the knives are held tightly to their mounting surfaces.
Once a cutting edge becomes dull, the knife can simply be reversed, placing its opposite edge into the cutting position. Unlike conventional knife systems, no readjustment is required after the knives have been reversed. After the second edge has become dull, the user can resharpen each edge one time. When the knives are resharpened, the bed knives must be readjusted to provide the proper clearance for cutting.
The knife and clamping arrangement disclosed herein dramatically improve the economics of attaining longer lasting knife edges, reduces machine downtime, reduces labor costs of changing knives, and improves granulate quality by minimizing fines.
The knife is smaller and requires less material to manufacture. The reduced size is attained because a separate clamp bar is used to retain the knife as opposed to bolting the knife down directly. The small configuration permits the economical use of exotic materials which will stay sharp much longer.